Rietveld analysis of the modulated structure in the superconducting oxideBi2(Sr,Ca)3Cu2O8+x

Abstract

The modulated structure of the high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Bi}}_{2}$(Sr,Ca${)}_{3}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8+\mathit{x}}$ was analyzed with a newly developed Rietveld refinement program for modulated structures. The structure refinement was made by the simultaneous refinement method using x-ray and time-of-flight neutron-diffraction data. It has a one-dimensionally modulated structure with N Bbmb/11\ifmmode\bar\else\textasciimacron\fi{}1, a=5.3957(3), b=5.3971(3), c=30.649(1) \AA{}, and k=0.2118(1)${\mathbf{b}}^{\mathrm{*}}$. Atoms in Bi-O, Sr-O, Cu-O, and Ca layers are all greatly displaced from their average positions. In particular Bi atoms are displaced mainly along the b axis, forming Bi-condensed and Bi-dilute regions. The characteristic feature of the structure is the existence of extra oxygens, giving x=1.0, in the Bi-dilute region of the Bi-O layer. Bi has a distorted rocksalt-type oxygen coordination with four short Bi-O bonds in the Bi-condensed region and a deformed square pyramidal coordination with five Bi-O bonds in the Bi-dilute region. The apical oxygen of a ${\mathrm{CuO}}_{5}$ pyramid, which joins Cu to Bi, moves along with Bi, distorting the pyramid considerably.